Chapter 8

Chemical Reactions and Equations

Chemical reactions….

• Process by which one or more substances are changed into one or more different substances

• described by a chemical equation

4 Signs of a Chemical Reaction:

1. Evolution of heat and light

2. Production of a gas

3. Formation of a precipitate (solid produced as a result of a chem rxn in a sol’n)

4. Color change

Chemical equation

• Represents the identities and relative amounts of the reactants and products in a chemical reaction

• (NH4)2Cr2O7(s) ---> N2(g) + Cr2O3(s) + 4H2O(g)

• Characteristics:– represents the known facts– contains correct formulas for the reactants & products– satisfies the law of conservation of mass (same # and

kind of atoms on both sides of the equation)

(NH4)2Cr2O7(s)---> N2(g) + Cr2O3(s) + 4H2O(g)

• Formula equation: uses symbols/formulas to represent the reactants and products

• Reactants

• products

• coefficient (understood to be “1” if no # appears)

• symbols (p.246)

ammonium dichromate --> nitrogen + chromium(III) oxide + water

• Word equation: (previous ex.)

• only fulfills ONE characteristic of a chem eq (represents the facts)

• 1st step in writing a balanced chemical equation

Steps to write a balanced chemical equation:

1. Write the word equation.

2. Convert the word equation into a formula equation.

3. Include symbols, where known.

4. Balance the equation. NEVER CHANGE A SUBSCRIPT IN BALANCING EQUATIONS!

Cu + H2SO4 CuSO4 + SO2 + H2OCu

H

S

O

Cu + H2SO4 CuSO4 + SO2 + H2O

Cu +2H2SO4 CuSO4 + SO2 + 2H2O

Beaker Breaker

• Balance the following equation

___Fe + ___H2O ____Fe3O4 + ____H2

SiF4 + H2O H2SiF6 + H2SiO3

Si

F

H

O

SiF4 + H2O H2SiF6 + H2SiO3

3SiF4 + 3H2O 2 H2SiF6 + H2SiO3

Solutions of lead(II) nitrate and sodium hydroxide react to produce a lead(II) hydroxide precipitate and an aqueous solution of sodium nitrate.

Pb(NO3)2(aq) + 2NaOH(aq)

Pb(OH)2(cr) + 2 NaNO3(aq)

Solid rubidium oxide reacts with water to produce an aqueous solution of rubidium

hydroxide.

Rb2O(s) + H2O(l) 2RbOH(aq)

Lithium reacts with a hydrochloric acid solution to produce an aqueous solution

of lithium chloride and hydrogen.

2Li(cr) + 2HCl(aq) 2LiCl(aq) + H2(g)

In the presence of an electrical current, sodium hydroxide can be produced from the reaction of a sodium chloride solution with water. Hydrogen and chlorine gases are the other products.

2NaCl + 2H2O 2NaOH + H2 + Cl2

Beaker Breaker

Balance the following equation:

______Na + _______H2O _____H2 + ______NaOH

Aluminum sulfate is produced when aluminum hydroxide reacts with sulfuric acid. Water is the other product.

2Al(OH)3 + 3H2SO4 Al2(SO4)3 + 6 H2O